In sewing machines, an upper thread is inserted in a needle, while a lower thread is retained in a hook. An upper shaft driving a needle bar and a lower shaft driving the hook are coupled with each other through a toothed belt. That is, when the upper shaft is driven by the drive force, etc., of a motor, the lower shaft also rotates, and the needle and the hook are relatively operated. Sewing machines catch, through the tip of the hook, the thread loop formed by the upper thread when the needle rises after falling to the needle bottom dead center, and form a stitch by intertwining the upper thread and the lower thread with each other.
A wobbling mechanism which is provided on the upper shaft swings the needle bar so as to intersect a cloth feeding direction, and thus sewing machines are capable of forming zig-zag stitches. When this wobbling mechanism is controlled and the swinging level of the needle bar and the timing thereof are adjusted, various complex sewing, such as a whipstitch, a pattern stitch, and a letter stitch, is realized.
When a complex sewing is performed, if the swing of the needle bar becomes large, the position of the needle frequently changes. Hence, the relative positional relationship between the needle and the hook changes, and the timing at which the needle and the hook relatively operate are changed together with the change in the relative positional relationship. When this timing change exceeds an allowable range, it becomes difficult to form a stitch.
Hence, according to conventional sewing machines, the swing level of the needle bar is limited so as to set the change in relative operation of the needle and the hook within a range that enables a formation of a stitch. Alternatively, two idlers forming an idler unit are caused to contact the toothed belt which couples the upper shaft and the lower shaft (see, for example, JP2008-264500 A). When the idler unit is driven so as to operate together with the swing of the needle bar in the case of, for example, zig-zag stitches, to change the belt length of the toothed belt at the tensioned side, thereby controlling the timing of the relative operation of the needle and the hook. Through such a control, the gap in the timing of the needle linked with the swing of the needle bar, and the hook is corrected, so as to enable a formation of a stitch even if the swing width is large.
In general, toothed belts are formed so as to have a certain slack due to the workability at the time of attachment and the safety reason. The idlers absorb such a slack at the loosen side of the toothed belt to suppress a detachment of the belt, thereby transmitting force stably. Conversely, JP2008-264500 A includes an idler unit having two idlers movable on a rotation plane of the toothed belt. This idler unit arbitrarily moves the slack between the tensioned side and the loosen side of the toothed belt, thereby controlling the timing of the needle and the hook.
However, the movement of the idlers partially detracts a function of absorbing the slack and making the tension of the toothed belt stable. When the idlers move, the absorbing level of the slack of the toothed belt by the idlers changes. Hence, the tension of the toothed belt is changed, causing vibrations and noises when power is transmitted, and a detachment of the belt.
According to conventional technologies, in order to address such problems, cam faces that drive respective two idlers are provided for a drive source which drives the idler units. The two idlers are operated differently so as to appropriately change the distance between the idlers, thereby maintaining a constant absorbing level of the slack of the toothed belt when the idlers move. The change in the tension of the belt is addressed by moving the idler unit as explained above.
However, the change in the tension of the toothed belt upon movement of the idler unit is affected by, for example, a variability of the component dimension, and a variability of the positional relationship between the upper shaft and the lower shaft, and between those and the idler unit. Hence, the way of moving the idler unit while changing the distance between the idlers and the change level of such a distance vary depending on a sewing machine. Thus, a fine adjustment mechanism to adjust the distance between the idlers is provided, but because of the nature of the adjustment of a fine change in tension when the toothed belt is rotated, the work is difficult and it is not practical in view of a mass-production.
The present invention has been made to address the above-explained problems of the conventional technologies, and it is an objective of the present invention to provide a sewing machine provided with a mechanism which can stably control a gap in the timing of a needle and a hook caused by a swing of a needle bar without a complex adjustment work.